The invention relates to a belt tensioning device for a belt drive having at least two belt pulleys and a continuous belt, and a torsion spring assembly.
A typical belt drive application is the drive of auxiliaries of an internal combustion engine, wherein a first belt pulley is positioned on the crankshaft and drives the belt drive, with further belt pulleys being positioned on auxiliaries such as a water pump, generator, air conditioning compressor etc. and being driven by the belt drive. In the direction of rotation, behind the driving belt pulley, there exists a slack strand whose slack has to be balanced by a tensioning roller to ensure that the belt does not slip off the belt pulleys. The belt length changes in the course of the operating period and under the influence of temperature, so that the tensioning roller has to be resiliently displaceably held in guiding means or, preferably, it has to oscillate at a resiliently suspended tensioning or rocker arm.
In the case of belt drives for driving auxiliaries of an internal combustion engine, which include a starter generator, between the engine operation on the one hand and the starter operation on the other hand, there occurs a change between the pulling strand and the slack strand on both sides of the belt pulley of the starter generator. In such a case, it is necessary to provide spring-loaded tensioning rollers for both the above-mentioned strands. One tensioning roller is effective at the slack strand under spring force, whereas the other one is optionally pushed back by the tensioned pulling strand against the spring force of same.
As a rule, the pre-tensioning forces acting on the tensioning rollers are applied by sprint assemblies which are positioned in the region of the bearing of a tensioning arm and contain stirrup springs, i.e. helical or hairpin springs with two radially projecting stirrups in the case of which there can occur a change in angle of the two projecting stirrups relative to one another against resilient spring forces. The stirrup spring either contracts or widens. Spring units of this type require a relatively large installation diameter which is not available in all applications.
From DE 39 12 944 A1, there is know a belt tensioning device wherein the spring element is provided in the form of a rotary bar spring which is composed of a plurality of rotary bars and whose longitudinal axis extends parallel to the axis of rotation of the tensioning roller. The tensioning roller is supported in a tensioning lever which is arranged at the free end of the rotary bar spring. The fixed end of the rotary bar spring is firmly clamped into holding means provided with bolting means for being fixed to a fixed housing. In order to prevent the rotary bar spring from bending under load, it is arranged in a guiding tube, with the rotary bar spring and the guiding tube, at the end carrying the tensioning lever, both being connected in a rotationally fixed way to the tensioning lever. The guiding tube, in turn, is rotatably supported in the engine block.
Thus, there exists a need for a compact belt tensioning device suitable for standard drives with an unchanging driving pulley and belt drives with changing driving pulleys.